Method for preparing magnetic compositions



United States Patent METHOD FOR PREPARING MAGNETIC COMPOSITIONS Henry L.Crowley, South Orange, N. J., assignor, by mesne assignments, to HenryL. Crowley & Company, Inc., West Orange, N. J., a corporation of NewJersey No Drawing. Application November 18, 1952, Serial No. 321,291

Claims. (Cl. 252-625) This invention relates to magnetic compositionsand articles formed therefrom, and has for its object the provision ofcertain improvements in the method of making the same. Moreparticularly, the invention contemplates certain improvements in thosemethods of making magnetic compositions, and articles formed therefrom,described in my United States Letters Patent No. 2,575,099, patentedNovember 13, 1951, in my copending patent application Ser. No. 226,545,filed May 15, 1951 and in the copending patent application of Arthur M.Hossenlopp and myself Ser. No. 230,706, filed June 8, 1952. Morespecifically, the present invention involves certain improvements in thepreparation of the initial oxide mixture for the pre-firing or firststage heat-treatment in those methods of making magnetic compositions.

The methods disclosed in the aforementioned patent and patentapplications involve subjecting an initial oxide mixture composed of atleast 50% by weight of ferric oxide and containing electrical andmagnetic modifying metal oxides (e. g. zinc oxide and nickel oxide,respectively), to a two-stage heat-treatment. The first heattreatment iscarried out at a temperature of l000 to 1450 C. and the secondheat-treatment is usually carried out at a somewhat higher temperaturethan that of the first heat-treatment. The properties of the magneticcomposition are influenced by the heat-treatment temperatures, and therespective temperatures of the two heat-treatments are determined (byexperience or test) to impart to the composition the optimum electricaland magnetic prop-- erties for its intended use. The environments inwhich the initial oxide mixture and the product of the firstheattreatment are respectively heat-treated are so controlled thatferrous oxide is present in the resulting product at the conclusion ofthe second heattreatment, and the product is gradually cooled through atleast the greater part of the temperature range in which ferrous oxidespontaneous ly dissociates into metallic iron and magnetite. Throughoutthis specification and the appended claims magnetic composition meansthe final and finished composition and includes articles made therefrom.

I have now discovered that the electrical and magnetic properties of themagnetic compositions of the aforementioned patent and patentapplications can be usefully enhanced by substantially deaerating anddensely compacting the intimately-mixed initial oxide mixturepreparatory to the pie-firing or first-stage heat-treatment. Based onthat discovery, the present invention involves compacting and densifyingthe initial oxide mixture under considerable pressure after andpreferably while the mixture is subjected to a relatively high vacuum.The thuscompacted and densified mixture, in the shape or form resultingfrom compacting, is then subjected to heattreatment at a temperaturewithin the range of 1000 to 145 0 C., and the resulting heat-treatedproduct is crushed and ground to a relatively fine powder. 7

Compacting of the initial oxide mixture may be done by pressure alone,as for example by briquetting in a mold or press, by extrusion, or bypressure followed by extrusion, all under evacuated conditions. A verycon siderable pressure is used in compacting, since the purpose is todensity the deaerated mixture a-s greatly as practical. To this end, acompacting pressure of at least 5000 pounds per square inch is employed,and preferably the compacting pressure is about 20,000 to 40,000 poundsper square inch. Moreover, it has been found, for reasons not clearlyunderstood, that extrusion, or a combination of pressure and extrusion,imparts superior physical and electncal properties to the magneticcomposition, as contrasted with briquetting alone. Compacting anddensifymg may advantageously be carried out by extruding the deacratedmixture in the form of bars or rods approxi mately one-half /2) inch indiameter under a pressure approximating 20,000 pounds per square inch.The extruded bars break off in lengths of about 3 to 4 inches, suitablefor handling before, during and following prermg.

Prior to compacting, the initial oxide mixture is subected to arelatively high vacuum, sufficient to substantially deaerate themixture, so that the compacted and densified mixture is substantiallydevoid of air pores. The high vacuum should be maintained duringcompactmg to avoid re-entrance of air into any of the already evacuatedpores of the mixture. In practice, .it is usually preferable to carryout the steps of evacuation and compacting simultaneously. The vacuumshould be as high as practical, and of the. order of at least 25 inchesof mercury, and preferably as high as 28 inches of mercury, and evenhigher if practical.

In my aforementioned Patent No. 2,575,099, I have describedagglomerating the initial oxide mixture preparatory to pre-firing, butsuch agglomeration was primarily for convenience in handling themixture, and was not carried out under evacuated conditions. In latercommercial practice of the method of that patent, it was frequentlycustomary to pre-fire the initial oxide mixture in granular form, thatis as a more or less loose powder. The discovery upon which the presentinvention is based was made in the course of an investigation of certaindeficient and unpredictable physical characteristics, both macroandmicroscopic, which were frequently present in structures of'the magneticcomposition. It now appears that when the initial oxide mixture ispre-fired in the form of a more or less loose powder, or in the form ofordinary agglomerates, the inevitable presence of innumerable air spacesWithin the mass of the mixture results in a product which, even aftergrinding, is so permeated with internal air pores as to exercise a.deleterious effect not only upon the physical but also upon the magneticand electrical characteristics of the magnetic material. Irrespective ofthe degree of subsequent grinding, it appears impossible to sufiicientlyeliminate internal air pores in the pre-fired product to prevent thesedeleterious effects. On the other hand, when the initiai oxide mixtureis pre-fired in the form of the deaerated and densely compacted shapesof the invention, the pre-fired product is substantially solid and forall practical purposes is substantially devoid of internal pores. Evenafter grinding, as well as throughout all subsequent processing steps,this characteristic freedom from internal pores or voids is maintained,with all of its attendant advantages upon the magnetic and electricalproperties of the magnetic composition.

In addition to substantially eliminating internal air pores, practice ofthe invention results in other benefits and advantages. When the initiaioxide mixture is pre fired in a more or less loose powdery form,substantial loss of certain constituents of the mixture, notably zincoxide, takes place during pre-firing. Since these losses are not alwaysuniform, the chemical composition of the prefired product will vary,depending upon the losses, and such variations in chemical compositioncause undesirable variations in the magnetic and electrical propertiesof the magnetic composition. By deaerating, compacting and densifyingthe initial oxide mixture, such losses during pre-firing becomepractically negligible. Additionally, the heat-treatment of the initialoxide mixture in the form of deaerated and densely compacted shapespromotes uniform and complete chemical reaction between the metal oxideconstituents of the mixture whereby the heattreated product has greateruniformity of chemical com? position, and hence the magnetic andelectrical properties of the magnetic material are more uniform.Further,

heat-treatment of the initial oxide mixture in the form of deaerated anddensely compacted shapes imparts certain physical propertiesv to theheat-treated product which beneficially influence shrinkage during thesecond-stage of heat-treatment. Taken in their entirety, theseadvantages enable and insure repeated and continuous commercialproduction of magnetic compositions of more consistent-- ly uniformmagnetic and electrical properties than has heretofore been attained.

Heat'treatment of the deaerated and densely compacted shapes is carriedout in any suitable type of kiln or furnace at a temperature betweenabout l000 C. and about 1450 C., depending upon the desiredcharacteristics of the magnetic material. The temperature ofheat-treatment is preferably somewhat above the softening temperature ofat least one of the constituents of the initial oxide mixture, but nothigh enough to result in an appreciable vapor pressure. In order todefinitely allow the interior of all the shapes undergoingheat-treatment to reach a uniform elevated temperature before theexteriors of the shapes shrink sufficiently to close the pores, heatingis leveled-off at a temperature substantially lower (e. g. 50200 C.lower) than the ultimate temperature of heat-treatment, and maintainedat the leveling-off temperature until the entire mass of the shapes haveuniformly attained that temperature, whereupon the shapes are graduallyheated to the ultimate temperature of heat-treatment. In practice, theshapes may be permitted to shrink about 2 to 4% before reaching theleveling-off temperature. At the completion of heat-treatment, shrinkageof the shapes may amount to from 10 to 20%. In a typical heattreatmentcycle, the shapes are progressively heated to a levelling-offtemperature of about 10004250 C., in about 2 hours, are maintained atthat temperature for approximately 2 hours, and are then graduallyheated over a period of approximately 2 hours to the ultimatetemperature of heat-treatment of about 1200l350 C.

In practicing the methods of the aforementioned patent and applications,magnetic compositions of superior mag netic and electrical propertiesare produced when prefiring or first-stage heat-treatment is conductedunder conditions promoting the removal of some, but not all, of theoxygen content of the ferric oxide component of the initial oxidemixture. Although heating to a temperature within the aforementionedrange in an indifferent ambient atmosphere will effect a significantremoval of oxygen from the ferric oxide, it has been found generallypreferable to effect removal of a more substantial and definite amountof oxygen from the ferric oxide by a reducing environment such as thatprovided by a reducing ambient atmosphere or by a reducing agent presentin the initial oxide mixture, or by both. Reducing agents suitable forincorporation in the initial oxide mixture are preferably suchcarbonaceous materials as dextrin, starch, flour, cellulosic substancesetc., and may advantageously be added in amounts of from about 1 to 5%by weight of the initial oxide mixture.

In compacting and densifying the deaerated initial oxide mixture, it isdesirable to incorporate therein a binder and sufficient moisture toimpart plasticity. Preferably, the binder is one of the aforementionedcarbonaceous materials which serves additionally as a reducing agent. Amoisture (water) content of about by weight usually imparts the desiredamount of plasticity to the mixture. The deaerated and densely compactedshapes are dried in any suitable manner, as for example in an oven at atemperature slightly above 100 C. to eliminate moisture, and are thenpre-fired as hereinbefore described.

At the conclusion of the pre-firing or first-stage heattreatment, theheat-treated shapes are preferably quenched to minimize dissociation offerrous oxide into metallic iron and magnetite, since the presence ofmagnetic materials is detrimental in the subsequent grinding operations.Quenching is conveniently effected by directly discharging theheat-treated shapes from the furnace into water. The quenched shapes arethen crushed and ground, preferably to as uniform a size as practicable.Grinding should produce a relatively fine powder, say of an averageparticle size not exceeding 25 microns. However, in the preferredpractice of the aforementioned methods, much finer grinding isdesirable, preferably such that at least 75% of the ground powder is ofa particle size between 2 and 10 microns. Particles of 1 micron andsmaller are undesirable, since they present too great surface areas andtend to chemically impair the magnetic composition. On the other hand,too large a proportion of particles above 10 to microns in size isundesirable.

Particle size determinations may conveniently be made with aCenco-Sheard-Sanford Photclometer, obtained from the Central ScientificCompany, of Chicago. The determinations are made by allowing a portionof the ground product to settle through a transparent liquid of knownviscosity and depth. A beam of light is passed through the liquid and aphotoelectric cell arrangement records the amount of light passed in agiven time. As the particles settle through the liquid they interruptthe light. Since the particles settle in accordance with Stokes law, theamount of light that is cut off during a given time period is correlatedto the percentage of particles of a certain particle size or within aparticular range of particle size.

The finely ground product of the pre-firing or firststage heat-treatment(with added binder) is compacted. and pressed by any suitable apparatusinto shaped articles of the shape and size predetermined by the shapeand size of the final articles (e. g. a magnetic core), and the shapedarticles are subjected to the secondstage of heat-treatment orfinal-firing. Due allowance should be made for shrinkage duringfinal-firing. As a consequence of pre-firing the deaerated and denselycompacted initial oxide mixture in accordance with the presentinvention, shrinkage during final-firing is very considerably less thanoccurs when the initial oxide mixture is pre-fired in the form ofordinary agglomerates or a more or less loose powder. Heretofore,pressures of the order of 25,000 pounds per square inch have customarilybeen employed in compacting the product of pre-firing an initial oxidemixture in powdery form or in the form of ordinary agglomerates, andshrinkage during final-firing is about 15%. Shaped articles made fromproducts of the pro-firing practice of the invention and compacted underthe same pressure shrink less than 10% during final-firing.Alternatively, where a predetermined shrinkage during final-firing isdesired, a very considerably lower compacting pressure is employed withproducts of the pre-firing practice of the invention than with productspre-fired in powdery form or as ordi nary agglomerates. Thus, usingexisting dies designed for a shrinkage of 15% during final-firing, acompacting pressure of only 5,000 pounds per square inch is requiredwith products of the pro-firing practice of the invention as contrastedwith a compacting pressure of about 25,000 pounds per square inch withproducts prctired in powdery form or as ordinary agglomerates.

While the invention is of special advantage and particularly applicableto the methods of making the magnetic compositions described in theaforementioned patent and patent applications, it can be practiced withadvantage in the heat-treatment of any mixture of finely dividedmetallic oxides in the course of preparing magnetic materials therefrom.It is well known that air gaps (e. g. voids or pores) have a pronouncedeffect on the electrical and magnetic properties of magnetic materials,and that usually such effects are undesirable, particularly in magneticcores. By the practice of the invention, internal air pores arepractically non-existent in the heat-treated product, even aftergrinding to a relatively fine powder, and this absence of air poresremains characteristic of the product through all subsequent processingsteps in the production of the contemplated magnetic material. It thusbecomes possible to commercially produce by heat-treatments magneticcompositions or materials of uniform magnetic and electrical propertiesfrom finely divided mixtures of metal oxides, which may includesub-oxides, such as ferrous oxide, metallic constituents, such asmetallic iron, and the like, all in a finely divided state ofsubdivision.

I claim:

1. In a method of preparing a magnetic composition in which an initialoxide mixture composed of at least 50% by weight of ferric oxide andcontaining at least two other metal oxides capable of modifying themagnetic and electrical properties of the composition is subjected toheat-treatment at a temperature within the range of 1000 to 1450 C., theimprovement which comprises substantially deaerating the initial oxidemixture by subjecting it to a vacuum of at least 25 inches of mercury,compacting the deaerated mixture under a pressure of at least 5,000pounds per square inch into dense shapes substantially devoid ofinternal air pores, subjecting the thus-compacted and dense shapes toheat-treatment at a temperature within the aforementioned range, andgrinding the resulting heattreated product to a relatively fine powderin which at least 75 by weight is of a particle size between 2 andmicrons.

2. The improvement according to claim 1 in which the heat-treatedproduct is quenched in water directly followmg heat-treatment.

3. The improvement according to claim 1 in which the heat-treatingtemperature to which the shapes are subected is progressively raiseduntil the shapes have undergone a shrinkage of from 2 to 4% at aheat-treating tem perature substantially lower than the ultimatetemperature of heat-treatment, continuing heating of the shapes w1thoutfurther increase in the heat-treating temperature until they areuniformly heated throughout to the aforesa d lower heat-treatingtemperature, and then gradually ra1 s1ng the heat-treating temperatureuntil the shapes are uniformly heated throughout to the ulimatetemperature of heat-treatment.

4. In a method of preparing a magnetic composition in which an initialoxide mixture composed of at least 50% by weight of ferric oxide andcontaining at least two other metal oxides capable of modifying themagnetic and electrical properties of the composition is subjected toheattreatment at a temperature within the range of 1000 to 1450 C., theimprovement which comprises substantially deaerating the initial oxidemixture by subjecting it to a vacuum of at least 28 inches of mercury,compacting the deaerated mixture under a pressure of at least 5,000pounds per square inch into dense shapes substantially devoid ofinternal air pores, subjecting the thus-compacted and dense shapes toheat-treatment at a temperature within the aforementioned range in thecourse of which the heat-treating temperature is progressively raiseduntil the shapes have undergone a shrinkage of from 2 to 4% at aheat-treating temperature from 50 to 200 C. lower than the ultimatetemperature of heat-treatment, continuing heating of the shapes withoutfurther increase in the heattrcating temperature until they areuniformly heated throughout to the aforesaid lower heat-treatingtemperature, then gradually raising the heat-treating temperature untilthe shapes are uniformly heated throughout to the ultimate temperatureof heat-treatment, quenching the resulting heat-treated shapes in water,and grinding the quenched shapes to a relatively fine powder in which atleast 75 by weight is of a particle size between 2 and 10 microns.

5. In a method of preparing a magnetic composition in which an initialoxide mixture composed of at least 50% by weight of ferric oxide andcontaining at least two other metal oxides capable of modifying themagnetic and electrical properties of the composition is subjected toheat treatment at a temperature within the range of 1000 to 1450 C., theimprovement which comprises compacting and densifying theintimately-mixed initial oxide mixture under a pressure of at least5,000 pounds per square inch while maintained under a vacuum of at least25 inches of mercury, subjecting the thus-compacted and densified oxidemixture to heat-treatment at a temperature within the aforementionedrange, and grinding the resulting heattreated product to a relativelyfine powder in which at least 75% by weight is of a particle sizebetween 2 and 10 microns.

6. The improvement according to claim 5 in which the heat-treatingtemperature to which the compacted and densified oxide mixture issubjected is progressively raised until the compacted mixture hasundergone a shrinkage of from 2 to 4% at a heat-treating temperaturesubstantially lower than the ultimate temperature of heat-treatment,continuing heating of the compacted mixture without further increase inthe heat-treating temperature until the compacted mixture is heatedthroughout to the aforesaid lower heat-treating temperature, and thengradually raising the heat-treating temperature until the compactedmixture is uniformly heated throughout to the ultimate temperature ofheat-treatment.

7. The improvement according to claim 5 in which the heat-treatedproduct is quenched in water directly following heat treatment.

8. The improvement according to claim 6 in which the heat-treatedproduct is quenched in water directly following heat-treatment.

9. In a method of preparing a magnetic composition in which an initialoxide mixture composed of at least 50% by weight of ferric oxide andcontaining at least two other metal oxides capable of modifying themagnetic and electrical properties of the composition is subjected toheattreatment at a temperature within the range of 1000 to 1450 C., theimprovement which comprises compacting the initial oxide mixture intodensified bars about onehalf inch in diameter by extruding the mixtureat a pressure of at least 5,000 pounds per square inch and whilemaintained under a vacuum of at least 25 inches of mercury, subjectingthe compacted and densified bars to heat-treatment at a temperaturewithin the aforementioned range, and grinding the resulting heat-treatedbars to a relatively fine powder in which at least by weight is of aparticle size between 2 and 10 microns.

10. In a method of preparing a magnetic composition in which an initialoxide mixture composed of at least 50% by weight of ferric oxide andcontaining at least two other metal oxides capable of modifying themagnetic and electrical properties of the composition is subjected toheat-treatment at a temperature within the range of 1000 to 1450 C., theimprovement which comprises compacting the initial oxide mixture intodensified bars about one-half inch in diameter by extruding the mixtureat a pressure approximating 20,000 pounds per square inch and whilemaintained under a vacuum of at least 28 inches of mercury, subjectingthe compacted and densified bars to heat-treatment at a temperaturewithin the aforemen tioned range in the course of which theheat-treating temperature is progressively raised until the bars haveundergone a shrinkage of from 2 to 4% at a heat-treating temperaturesubstantially lower than the ultimate temperature of heat-treatment,continuing heating of the bars without further increase in theheat-treating temperature until they are uniformly heated throughout tothe aforesaid lower heat-treating temperature, then gradually raisingthe heat-treating temperature until the bars are uniformly heatedthroughout to the ultimate temperature of heat-treatment, quenching theresulting heat-treated bars in water, and grinding the quenched bars toa relatively fine powder in which at least 75% by weight is of aparticle size between 2 and 10 microns.

Fessler Jan. 26, 1937 Lecuir June 6, 1950 Crowley Nov. 13, 1951

1. IN A METHOD OF PREPARING A MAGNETIC COMPOSITION IN WHICH AN INITIALOXIDE MIXTURE COMPOSED OF AT LEAST 50% BY WEIGHT OF FERRIC OXIDE ANDCONTAINING AT LEAST TWO OTHER METAL OXIDES CAPABLE OF MODIFYING THEMAGNETIC AND ELECTRICAL PROPERTIES OF THE COMPOSITION IS SUBJECTED TOHEAT-TREATMENT AT A TEMPERATURE WITHIN THE RANGE OF 1000 TO 1450* C.,THE IMPROVEMENT WHICH COMPRISES SUBSTANA VACUUM OF AT LEAST 25 INCHES OFMERCURY, COMPACTING THE DEAERATED MIXTURE UNDER A PRESSURE FO AT LEST5,000 POUNDS PER SQUARE INCH INTO DENSE SHAPES SUBSTANTIALLY DEVOID OFINTERNAL AIR PORES, SUBJECTING THE THUS-COMPACTED AND DENSE SHAPES TOHEAT-TREATMENT AT A TEMPERATURE WITHIN THE AFOREMENTIONED RANGE, ANDGRINDING THE RESULTING HEATTREATED PRODUCT TO A RELATIVELY FINE POWER INWHICH AT LEAST 75% BY WEIGHT IS OF A PARTICLE SIZE BETWEEN 2 AND 10MICRONS.